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High temperature thermal desorption (HTTD) is a process in which excavated contaminated materials are heated in a chamber, thereby volatilizing water, organic contaminants and certain metals. HTTD involves temperatures ranging from 315 °C to 538 °C (600 °F to 1,000 °F), while low temperature thermal desorption (LTTD) involves temperatures ranging from 90 °C to 315 °C (200 °F to 600 °F). Higher temperatures used in HTTD facilitate removal of semi-volatile organic compounds (SVOCs) (<0.01 mm Hg vapour pressure). Higher temperatures, however, may also change the physical properties of the soil, and consequent loss of organic matter may inhibit biological activity.
In contrast to incineration, the operating temperatures and residence times in a thermal desorption system are designed to volatilize selected contaminants without oxidizing them. Thermal desorption treatment is not designed to destroy organic.
Treatment of contaminated soil using a thermal desorption system requires excavation and transportation of the soil to the reactor, where it is heated to a predetermined temperature. Two common thermal desorption reactor designs are the rotary dryer and the thermal screw. Throughput rates can vary from less than 5 to 10 to approximately 50 metric tons per hour depending on the type of soil and treatment unit. Ex situ thermal remediation systems use relatively large amounts of energy to heat the excavated soil. The treated soil can be used on-site as backfill if it meets regulatory requirements, or it may be disposed of off-site.
A gas or vacuum system transports the vaporized water and contaminants to an air emission treatment system, where particulates and contaminants are removed.
HTTD systems may include:
Captured vapours including water vapour and volatile organic compound (VOC) vapours require treatment for the removal of particulates and contaminants. Particulate removal equipment may be wet scrubbers or fabric filters. Condensation or adsorption (for example through GAC) equipment may be used for contaminant removal. Alternatively, the contaminants in the off-gas may be destroyed in a thermal oxidation system, which can be operated flameless, with a direct flame or as a catalytic oxidation system. A carrier gas or vacuum is used to transport water and VOC vapours to the gas treatment system.
The method relies on traditional, commonly available civil/earthworks construction equipment and methods for the excavation component. Commercial and transportable units are available for the treatment component. Depending on the soil throughput rates, units may be mounted on one to five trailers.
Notes: Treatability tests are recommended to determine the efficiency of thermal desorption for removing various contaminants at various temperatures and residence times.
Remote sites are prone to high mobilization and on-site monitoring costs, limited equipment availability and short work windows.
Since it requires large and complex equipment and high energy consumption, thermal desorption is not well adapted for northern and remote environments.
Depending on the volume of the soil requiring treatment, the treatment plant may be in operation from weeks to months.
There are no major long-term considerations related to thermal treatment systems. If the treated soil is used as backfill, minor long-term considerations may include changes to the geotechnical properties of the soil or changes to the organic content of the soil due to the decomposition of soil components during heating.
Control and treatment of air emissions from thermal desorption operations are an extremely important consideration. Generation of off-gases, containing dioxins and furans, and/or halogenated acids can occur during thermal desorption treatment of soil with halogenated compounds. The system should be designed, operated and maintained to prevent the emission of metals, polycyclic aromatic hydrocarbons (PAHs) or dioxins/furans.
Application examples are available at these addresses:
This technology is capable of reducing final contaminant concentrations to below 5 mg/kg for target compounds. HTTD costs in Canada are competitive with landfills or biological treatment (CSMWG, 2005).
The time required to complete cleanup of a “standard” 18,200-metric ton (20,000-ton) site using HTTD is just over 4 months (Reference).
Composed by : Josée Thibodeau, M.Sc, National Research Council
Updated by : Martin Désilets, B.Sc., National Research Council
Updated Date : March 1, 2008
Latest update provided by : Marianne Brien, P.Eng., Christian Gosselin, P.Eng., M.Eng., Golder Associés Ltée
Updated Date : March 31, 2018